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Influence of mercury on hydrogen overvoltage on solid metal electrodes—III. Adsorption and evolution of hydrogen on poisoned platinum
Electrochimico
Acra.
1973,Vol. 18,
pp. 45-54.
Pergamon Press. Printed in Great Britain.
INFLUENCE OF MERCURY ON HYDROGEN OVERVOLTAGE ON SOLID METAL ELECTRODES-III. ADSORPTION AND EVOLUTION OF HYDROGEN ON POISONED PLATINUM J. VONDRAK and J. BALEJ Institute
of Inorganic
Chemistry,
Czechoslovak
Academy
(Received 17 January
of Sciences, Prague, Czechoslovakia
1972)
Abstract-The adsorption capacity for hydrogen as well as the exchange cd of hydrogen evolution and the double-layer capacitance of a platinum electrode depend on the degree of poisoning by mercury. Mercury electrodeposited with simultaneous hydrogen evolution on smooth platinum forms droplets, the curvature radius of which is of the order low6 cm, and for complete inhibition of hydrogen adsorption, an amount equivalent to several monolayers of mercury is necessary. By submerging a pure platinum electrode in a dilute solution of mercuric compounds without external polarization, a monolayer is formed and one atom of mercury replaces just one atom of hydrogen. On platinum-black electrodes, electrodeposited drops of mercury cover the orifice of the pores and one mercury atom shields on an average 2-3 active sites of the electrode surface.
Hydrogen evolution on solid metal electrodes is inhibited by the presence of small amounts of mercury on the electrode surface. According to previous papers[l, 21 amounts of mercury as great as 4 monolayers are necessary for complete poisoning of a smooth platinum electrode. Oh the contrary, Shlygin[3], Khomtshenko et a&&7], and Shashkina and Kulakova[fl-101 found that one atom of mercury is able to displace several atoms of hydrogen from the surface of black platinum and palladium electrodes. To explain this contradiction, the influence of the amount and the manner of deposition of mercury on hydrogen evolution, hydrogen adsorption, and the double-layer capacitance of both black and smooth platinum electrodes was studied. The results of this investigation are the subject of the present paper. EXPERIMENTAL Apparatus,
electrodes
TECHNIQUE
and chemicals
The experiments were carried out in a glass vessel with three compartments. The compartment for the investigated electrode contained only 6 ml of solution saturated either by nitrogen or by hydrogen. It was separated from the other compartments by means of stopcocks. The investigated, auxiliary, and reference electrodes were made from 5 x IO x 0.1 mm Pt foil and were welded to Pt wire, which was sealed in a glass holder with a ground joint. The reference
electrode was covered by platinum black and permanently saturated with hydrogen. Before each series of experiments, the smooth electrodes were successively purified in carbon tetrachloride, nitric acid (about 50 per cent), hydrochloric acid (about 30 per cent) and redistilled water. Before each run, the electrode was cleaned electrochemically by repeated polarization to f1200 mV and 0 mV(he) for a total time of 3 s. From hydrogen adsorption a roughness factor, measurements, f = 6.0 k O-3, was found for electrodes prepared in this way. Several electrodes were covered by platinum black in a solution containing 25 g HZPtCls and 0.08 g Pb(CH3C00)2 in 100 ml Hz0 using a cd of 0.03 A/cm2 for 30 min. Then the electrodes were leached severai days in redistilled water. From hydrogen adsorption measurements in pure 1 N KOH, roughness factors of 223, 315, 325 and 1050 were found. Properties of all the electrodes are summarized in Table 1. Both the hydrogen and the nitrogen used in the electrolytic vessel were purified by means of deoxidation catalysts, silica gel, and cooling with solid carbon dioxide. Both gases were let into the vessel through all-glass and elastic tube. No grease was used for the ground joints. Concentrated sulphuric acid (purity p.a.) and a 50 per cent solution of potassium hydroxide (purity for semiconductors) were used for solution preparation. Both H2S04 and KOH were diluted to a 45
J. VONDRAKAND J. BALEJ
46
Table 1. Properties of individual electrodes used in experiments
Electrode
Electrolyte solution
smooth4
1 NKOH
Roughness factor f
Exchange cd mA/cm’ “* io 10
Double-layer capacitance pF/cm’ c (3
0.48
0.08
85.7
8.7
1.74
110
18.3
14.3
6.0 i- O-3 1 nH2S04 black I
1 NKOH
223
20
0.09
3460
15.5
black II
1NKOH
315
15
O-048
4500
14.3
black III
1NKOH
258
10.5
o-047
5400
21.0
black IV
I N HzS04
14000
13.0
1090
-
* i; = iO]f. t c’ = C/J $ average of five samples.
concentration of 1 N. Mercuric oxide was prepared by precipitation of mercuric nitrate solution by potassium hydroxide. The mercury was scrubbed in dilute nitric acid and distilled in vacuum before preparation of the mercuric nitrate solution. Carbon tetrachloride and nitric and hydrochloric acids were purified by distillation. Four-times distilled water was used for all purposes. Before each series of experiments, the electrolytic vessel was cleaned with a 10 : 1 mixture of concentrated sulphuric acid and a 30 per cent solution of hydrogen peroxide (purity p.a.) and then rinsed with water. The vessel was then filled with the solution, oxygen was removed from the compartment for the investigated electrode, and the solution was purified by pre-electrolysis, using an electrode similar to that investigated and a cathodic cd of 0-l mA/cmZ for 10 h. Electrode
trode was inserted in the measuring vessel. Mercury ions on the electrode surface were reduced to the metal in this way. Tt was found that the rate of poisoning is controlled by the diffusion of Hgz+ ions to the electrode surface. Evidence of this is shown in Fig. 1, in which the charge, Q, necessary for the oxidation of mercury on the electrode surface (see below) is plotted against the square root of time, t, for which the electrode was submerged in 1 N KOH containing 50 mg/l of Hg2+. If each mercury atom is adsorbed immediately after reaching the electrode surface, then the total amount of mercury adsorbed at time t is 2~(Dr#/~, where c is the btllk concentration of Hg2+ ions in the solution and D is their diffusion coefficient. If this
poisoning
Three different modes of poisoning were used for smooth platinum electrodes. Mode 1. A clean electrode was connected to the negative terminal of a constant current power supply and then it was submerged in an electrolytic vessel containing a mercury pool anode and a solution of 1 N potassium hydroxide saturated with mercuric oxide. After time sufficient for deposition of the required amount of mercury, the electrode was withdrawn from the vessel, rinsed with water and transferred into the measuring vessel. Mode 2. A clean electrode was submerged in a solution of 1 N KOH saturated with mercuric oxide, or in a mixture of one volume of this solution and two volumes of pure 1 N KOH. After rinsing with water, the electrode was connected to a potentiostat on which the reversible potential of the hydrogen electrode had been pre-set, and the elec-
Fig. 1. Dependence of the charge required for the oxidation of mercury on the electrode surface on the time of dipping of the electrode in 1 N KOH containing HgO. 0, I N KOH; 0,l N H,SO,.
Influence of mercury on hydrogen overvoltage on solid metal electrodes-III. amount is reduced to metallic mercury, Q is required for its oxidation, Q = 2zF~(Dt7r)“~,
then charge (1)
where z is the va!ency state of the mercury oxidation products. It follows from Fig. 1 that the amount of mercury expressed as the charge Q is proportional to 2/i as required by (1). Further, the slopes of this relationship are different for the oxidation of adsorbed mercury in acidic and alkaline media, ie, 10.5 x 10m5 and 22 x 1O-5 A s’/‘/cm’, respectively. Theoretical values based on the value D = 8.2 x 10m6 cm2/s for mercuric ion in dilute nitric acid[ll] are 8.1 x lo-’ and 16.2 x 10n5 A s’/‘/cm’ for z = 1 and z = 2, respectively. From the ratio of the experimental slopes and from their approximate agreement with the calculated values, two conclusions can be drawn. First, the oxidation of mercury is an oneelectron process in sulphuric acid, while it is a twoelectron process in KOH. Second, the poisoning of an unpolarized electrode in a dilute solution of Hg”+ in 1 N KOH is a diffusion-controlled process. Mode 3. A clean electrode was submerged in liquid potassium amalgam for 1 min, and an almost continuous film of mercury was formed on the surface. Electrodes covered by platinum black were poisoned only according to Mode 1. Experimental
l-5
of potential changes during each experimental run is schematically shown in Fig. 2. After cleaning the electrode by anodic pulses and reduction of oxygen layers on theelectrode (Fig. 2, Section 1), the electrode was poisoned in a separate vessel (Section 2) and transferred back to the measuring cell. The polarization curve of hydrogen evolution was then measured using a step-wise change of potential from 0 to -100 mV(he) in the case of smooth platinum electrodes and to - 60 mV in the case of platinum black electrodes (section 3). In this potential range, the rate of evaporation of mercury from the electrode surface is negligible[2]. Then the double-layer capacitance was measured by superimposing an ac voltage on the dc potential in the range from +300 mV to +500 mV (Section 4) and both ac potential and current were recorded on an X-Y recorder (ENDIM 2200/I) or on an oscilloscope (Tesla OPD 250), and the capacitance was calculated from the Lissajous figures. Finally, the amount of adsorbed hydrogen and mercury were measured from cathodic and anodic voltammetric curves obtained with a voltage of triangular wave-form generated by a low-frequency generator (ARITMA GNK). For all experiments, a potentiostat IP 410 B was used. All potentials are referred to the reversible hydrogen electrode in the same solution. Experiments were done at 20 f 1°C. RESULTS
routine
Preliminary experiments showed that the described experimental technique ensured that the electrode surface remained clean for at least 3 min after electrode purification by anodic and cathodic pulses. Therefore, an experimental routine was chosen so that all electrode parameters, ie the hydrogenevolution rate, the double-layer capacitance, the amount of adsorbed hydrogen and the amount of mercury were measured within 3 min. The programme
47
Hydrogen
adsorption
on poisoned
electrodes
In Fig. 3, a typical anodic branch of a voltammetric curve (the ascending part of section 5 in
--____________--____----------
I _________________
V o-5
0
-05
Fig. 2. Potential changes during one experimental run. 1, electrode cleaning, 2, poisoning in separate vessel; 3, measuring of hydrogen overvoltage; 4, capacitance measurement by superimposed a.c. voltage; 5, cathodic and anodic voltammetry for measuring the amounts of hydrogen and mercury. EA Vol. 18 No. 1-D
Potential Fig. 3. Anodic branch of the voltamrnetric curve of a .. smoorn Pt electrode in 1 N HzS04 at the scanning rate 3 v/s. Full line, clean electrode; dotted line, electrode partly poisoned by mercury.
48
J. VONDRAK AND J. BALEJ
0
2
I P
Fig. 4. Influence of the relative amount of mercury P on the degree of coverage by hydrogen 8, for smooth electrodes poisoned by Mode 1. l,l N KOH; 0,l N HzS04. Fig. 2) for both clean and partly poisoned smooth electrodes is plotted. In these experiments, a scan rate of 3 V/s was used. The curve for the clean electrode is in agreement with that given by Breiter[lZ]. In the presence of mercury on the electrode surface, the amount of adsorbed hydrogen is lower and the peak currents of both hydrogen waves are reduced approximately in the same ratio. Also, the growth of the oxygen layer in the potential range from +0*7 + 1.0 V(he) is slower on a poisoned electrode; this effect is more apparent at higher scanning rates. At potentials more positive than +1-O V, a new wave of mercury oxidation occurs. This wave was also described by Khomtshenko[l31. The amount of adsorbed hydrogen atoms, Q”, was evaluated as an average of the charges of both cathodic and anodic waves on voltammetric curves in the potential range from 0 to +O-4 V, and the degree of coverage by hydrogen was calculated as the ratio of the amount of hydrogen on poisoned (Q”) and clean (Qh) electrodes: 0 = QH/Q~ . The amount of mercury was calculated from the charge QoXabsorbed by the electrode if the potential increased from +0.7 to +l*O V. It was expressed as the ratio of the number of mercury atoms on a poisoned electrode to the number of hydrogen atoms on a clean electrode,
where QZXis the charge required for oxidation of a clean electrode in the same potential range, and the meaning of z is the same as in (1). Its value was taken as z = 1 for acidic solutions and z = 2 for alkaline solutions. For a complete monolayer of mercury, the relative amount of mercury, P = 1, provided that the number of atoms in the monolayers of mercury and hydrogen are the same.
P
Fig. 5. Influence of the relative amount of mercury P on the degree of electrode coverage by hydrogen 6, for smooth electrodes poisoned by Mode 2. .,I N KOH, 0,l N HzS04.
Fig. 6. Influence of the relative amount of mercury P on the degree of electrode coverage by hydrogen & for black electrodes poisoned by Mode 1. Full points, 1 N KOH; open points, 1 N H1S04. v, electrode 1; 0, electrode 2; A., electrode 3 ; 0, electrode 4.
Influence of mercury on hydrogen overvoltage on solid metal electrodes-III.
49
In Figs. 4, 5 and 6, the degree of surface coverage by hydrogen is plotted against the relative amount of mercury, P. In these pictures, the dotted lines, cor-
responding
to substitution
of one hydrogen
atom
by one atom of mercury, are given for comparison. If a smooth electrode is poisoned by Mode 1 (Fig. 4), then small amounts of mercury have almost no influence on hydrogen adsorption, so that several atoms of mercury are necessary for the substitution of one hydrogen atom. This ratio decreases with increasing amounts of mercury, but for complete inhibition of hydrogen adsorption, a large excess of mercury is required. The behaviour of a smooth electrode poisoned by Mode 2 is different (Fig. 5). For relative amounts of mercury, P < O-5, one atom of mercury can substitute just one atom of hydrogen. At higher relative amounts of mercury, poisoning becomes less effective and the ratio between the increment of mercury amount and the decrement of hydrogen coverage is slightly higher than 1, analogous to the previous case. Small amounts of mercury deposited on a platinum black electrode according to Mode 1 (Fig. 6) are a
very effective poison, since one atom of mercury substitutes two atoms of adsorbed hydrogen, if 8, > 0.5. With increasing amounts of mercury, the effectiveness of the mercury decreases; nevertheless, relative amounts, P, as low as 2 are sufficieht for complete inhibition of hydrogen adsorption on platinum black electrodes (ie for 0, -C 0.05).
Fig. 8. Influence of electrode coverage by hydrogen on the relative exchange cd of hydrogen evolution on smooth electrodes poisoned by Mode 2. is approximately proportional to the coverage by hydrogen. This is shown in Figs. 7 and 8 for smooth electrodes poisoned by Mode 1 and Mode 2 and in Fig. 9 for black electrodes poisoned by Mode 1. In these pictures, exchange cds are expressed as the ratio of the exchange cds on poisoned and clean electrodes under the same conditions.
Hydrogen overvoltage on poisoned electrodes Analogous to hydrogen adsorption, the hydrogenevolution rate also decreases with increasing amounts of mercury. The exchange cd for hydrogen evolution
Fig. 9. Influence of electrode coverage by hydrogen on the relative exchange cd of hydrogen evolution on black platinum electrodes.
Fig. 7. Intluence of electrode coverage by hydrogen on the relative exchange cd of hydrogen evolution on smooth electrodes poisoned by Mode 1. .,lNKOH; 0,l NH,SO,.
The poisoning of a smooth electrode by Mode 1 was found to be less effective than poisoning by Mode 2 (cf Figs, 4 and 5, and also previous papers [l, 21). Therefore, the inhibition of hydrogen evolution by mercury must be studied up to amounts of mercury much higher than that equivalent to one monolayer. The dependence of the polarization resistance and the exchange cd is plotted in Fig. 10 against the amount of mercury on the logarithmical
J. VONDRLKAND J. BALEJ
50
Capacitance of poisoned electrodes Amc~lgomated ___________
electrode ___a---
While the differential capacitance of smooth electrodes poisoned by Mode 2 was found to be independent of P in the whole studied range 0 < 8 < 1, within 5 per cent accuracy, the capacitance of both smooth and black electrodes poisoned by Mode 3 depends on the relative amount of mercury, as shown in Figs. 12 and 13 respectively. The capacitances are given in both figures in relative values as the ratio between the capacitance of poisoned and clean electrodes under similar conditions. In Fig. 12, the dotted lines indicate the value of the relative capacities for both clean electrodes and electrodes covered with mercury film (poisoning by Mode 3).
Fig. 10. Influence of the relative amount of mercury on the polarization resistance and the exchange cd of hydrogen evolution on smooth electrodes poisoned by Mode 1. .______ scale. Adsorption of mercury becomes undetectable by sweep voltammetry (0 G 0.03) if the amount of mercury increases to P = 2.7; this point is indicated by an arrow in Fig. 10. The dotted lines in Fig. 10 indicate both the limiting values of the hydrogenevolution rate, ie for a clean electrode and for an electrode poisoned by submerging in potassium amalgam (Mode 3) and covered with an almost continuous mercury film. According to this picture, even amounts of mercury as high as P = 100 do not suppress the hydrogen-evolution rate to the limiting rate found for electrodes covered with a mercury film. Figure 11 shows the dependence of the transfer coefficient /3 on the relative amount of mercury. Also in this picture, limiting values for both clean and mercury-covered electrodes are given.
Pure electrode
-0’0
.
z 0
Amlrlgamated
electrode
l
3
Fig. 12. Influence of relative amount of mercury on the relative electrode capacitance of smooth electrode poisoned by Mode 1.
I
1
P
Fig. 11. Influence of relative amount of mercury on the transfer coefficient B on smooth electrodes poisoned by Mode 1.
Fig. 13. Influence of the relative amount of mercury on the relative electrode capacitance of platinum-black electrodes. Symbols as in Fig. 6.
Influence of mercury on hydrogen overvoltage on solid metal electrodes-III. DISCUSSION According to Figs. 4,s and 6, the relative exchange cd of hydrogen evolution is approximately equal to the degree of coverage by hydrogen, Lel
10,pois e “. = -= i,, E~can
(3)
The total apparent cd on a partly poisoned electrode is the sum of the currents of hydrogen evolution on the clean and the poisoned parts of the electrode surface (supposing that the presence of mercury does not change the electrode surface area), i0. pOis= i,, P, . 0” + io, PfHpl* (1 - &I.
(4)
Here, io. Pt and io, ptHpXare the exchange cds on pure platinum and on platinum covered by mercury. For the relative current density of hydrogen evolution it follows that io,re,=(l-*)
-OH+%.
(4a)
According to Fig. 10, the ratio io, PfHpx/io,Pt = 7.4 x 10-4. Therefore. the last term in (4a‘l is negligible and this equation reduces to (3), as‘foknd experimentally. Equations (3) and (4a) are exactly valid if only one of the following conditions is fulfilled. Either the electrode surface is homogeneous and all centres have the same activity, or the surface is inhomogeneous, but different parts of the surface are poisoned to the same extent irrespective to their activity. The latter seems more probable as on a platinum electrode surface, at least two different types of active centres are present[l2, 141. As will be shown below, we suppose that, in the poisoning smooth electrodes by Mode 1, small drops of mercury are deposited. Owing to the different interfacial energies of the boundaries Pt/Hg, Pt/H, , and Pt/aqueous-solution, it seems probable that mercury drops occupy the concave parts of the surface, while hydrogen bubble formation is easier on the convex parts of the surface. Therefore, the mercury is initially deposited predominantly on parts of the surface that are less active for hydrogen evolution, and the small deflexion of the observed curve (Fig. 7) from (3) can be explained in this way. The exchange cd on an electrode covered by a mercury film (poisoning by Mode 3) is approximately 3 x lo-’ A/cm2 (Fig. lo), while the exchange cd on liquid mercury is 10-l” A/cmz[15]. Thus, a platinum electrode covered by mercury is by 6 orders of magnitude more active than pure mercury. Two explanations of this are possible. First, mercury dissolves platinum, forming amalgams with concentrations up to 0.06-0.1 per cent[16, 171, and the hydrogen overvoltage on platinum amalgam may be lower than that on mercury. Second, platinum is
51
incompletely wetted by mercury; the mercury film on the electrode surface is not continuous and contains small islets of bare platinum acting as hydrogenevolution centres. Actually, one can observe with the naked eye that bubbles of hydrogen occur on almost the same places on the surface. The relative exchange cd, 7-4 x 10e4, on mercury-covered platinum would therefore correspond to the fraction of uncovered platinum surface. Similarly we suppose that incomplete wetting of platinum by mercury is also important at lower mercury coverage on electrodes poisoned by Mode 1. In our previous papers[l, 21, we suggested a hypothesis according to which mercury deposited on a platinum electrode consists of small isolated droplets rather than of a continuous layer. This hypothesis seems sufficiently confirmed by the results of the present paper. Let us suppose for simplicity that the platinum electrode is covered by mercury drops of uniform size and with the shape of a spherical segment with radius of curvature, r, and wetting angle, 4 (Fig. 14). 1 cm2 of physical surface area contains n such droplets.
Fig. 14. Size and shape of a mercury drop on the electrode surface. Let tc = 1 - cos $.
(5)
Then, the volume of one drop is V = 1/3rrr3a2(3 - a), the surface of the spherical segment is S = Tar’ and the surface of the base of the drop is A = rrz * (2a - a’). Since the spherical surface of the drop is greater than its base, the total electrode surface area is increased by poisoning. Further, if d is the density of mercury and m. = 4.46. 10m7 g/cm2 is the amount of mercury required for a monolayer, the relative amount of mercury is given by
P=nY$. The degree of coverage by hydrogen equals part of the electrode not covered by mercury,
eH = I - d.
that (7)
J. VONDRAK AND J. BALEJ
52
Finally, the capacitance of the electrode is the sum of the capacitances of the free platinum surface and of the spherical surface of mercury so that the relative capacitance Gel is given by crc, = 1 - nA + F
P,
* ns,
(8)
where CHI and CP, are the specific capacitances of mercury and platinum respectively. Supposing that n is constant and that with increasing amounts of mercury, only the drop radius increases, from (6) and (7) it follows that
0
112
()
P=&- f
3-a . (2 _ a)3,2 * (1 - e”Y.
(9)
Similarly, if r is constant, and, with increasing amounts of mercury, new drops of constant radius are formed, then
(10)
,
Oo-
eti
f
Fig. 15. Relation between the electrode coverage by hydrogen and the relative electrode capacitance. Symbols as in Fig. 4 and 6. Data given by Khomtshenko for 1 N NaCl + O-01 N NaCl are presented by crosses.
By inserting the values of Q and Sin (8) we obtain the relation for the relative electrode capacitance, C,,, = F Pt . &+e. =B+([email protected]”
1
l-z-$-
a1 111)
This equation does not contain the drop radius r, and should be valid generally if the wetting angle is constant. In Fig. 15, the relative capacitance of both smooth and black electrodes are plotted against 6,. For comparison, data given by Khomtchenko[&7] are also plotted in the same picture. For both electrodes, (11) is valid if 8” > O-1. The parameter, B, for smooth electrodes is l-40, while for a platinum black electrode B is 0.45. From B = 1.40 for a smooth electrode, the wetting angle, 4 = 26.6” was established using (11) and (5) and CP, = 1.43 x 10e5 F/cm2 (found for a clean electrode, see Table l), and C,, = 1.80 x 1O-5 F/cm2 (found for an electrode poisoned by Mode 3). The behaviour of platinum-black electrodes will be discussed separately. From the theory of the condensation of liquids it follows that the formation of a new drop nucleus is more difficult than the growth of a drop previously formed. Hence, nucleation of drops should occur mainly at the beginning of the poisoning process. The number of mercury drops is probably influenced by the properties of the platinum surface and is approximately independent of the relative amount of mercury. Under these conditions, (9) should be valid and the dependence of P on (1 - &) should be a straight line with slope 3/2 on the logarithmic scale. As Fig. 16 shows, this is true only for mercury amounts giving P -=z1. For greater amounts of mercury, the slope decreases to l/2 and it increases again to higher values when most of hydrogen is removed from the electrode, viz for 0, < 0.2. The deviation of the observed curve from (9) in the medium range of mercury coverage is probably caused by the increasing number of drops in this range, while in the range of the highest amounts of mercury, drops of mercury fuse to greater lakes, which no longer have the shape of a spherical segment. Moreover, this more or less continuous mercury film also fills the micro-roughness of the electrode surface. For these reasons, not even (11) is valid and the relative capacitance decreases up to a value close to the reciprocal of the roughness factor (Fig. 15). From Fig. 16 it follows, for example, that 8” = 0.9 for P = 0.6. For this particular coverage, the number of drops, n = 2.2 x IO” cm-‘, and the drop radius, r = 1.92 x 10e6 cm, were calculated using 4 = 26.6” and (9) and (7). The vapour pressure on a curved mercury surface with this curvature radius is 1.325 times higher than that on a plane mercury surface. It was found experimentaIly that the mercury evaporation rate is
53
Influence of mercury on hydrogen overvoltage on solid metal electrodes-III.
P
I-
01
I
l-8, Fig. 16. The relation between P and (1 - 19,) for smooth electrodes poisoned by mode 1. The dotted line has slope 3/2.
1.14 f @05 times higher than that calculated for the equilibrium vapour pressure of pure mercury[2]. Hence, the vapour pressure elevation and theincrease in the mercury evaporation rate also coniirm the presented hypothesis of mercury-drop formation on a poisoned platinum surface. The difference between the theoretical and observed vapour-pressure elevation may be caused partly for the reasons discussed in the previous paper, partly by the fact that theevaporation rate was measured at higher amounts of mercury and therefore at greater radius of curvature of the mercury drops. According to a previous paper[l], the Tafel slope b of the hydrogen-evolution reaction on a posioned electrode is higher than on clean electrodes. This is in agreement with Fig. 11, which shows that, with increasing amounts of mercury, deposited by Mode 1, the transfer coefficient t?~decreases. This phenomenon can also be explained by the presence of mercury drops. The wetting angle, 4 on the platinum/ mercury/aqueous-solution boundary decreases if the potential increases to more negative values[ll], so that the mercury drops should spread and a greater fraction of the platinum surface would be shielded by the same amount of mercury than at less negative potentials. Therefore, the decrease of the transfer coefficient /3is only apparent. The behaviour of a black platinum electrode poisoned by Mode 1 is different. According to Fig. 6, one atom of mercury replaces two atoms of hydrogen. Similarly, Khomtchenko[4-71 found that one atom of mercury replaces 2-7 atoms of hydrogen on a platinum electrode with a specific area of surface 10,600. According to Shashkina and Kulakova[8-101 one atom of mercury replaces 3.1 atoms of hydrogen on black palladium electrodes with a specific surface area of 500. This phenomenon has usually been
explained by the heterogeneity of the electrode surface and by preferential poisoning of the most active centres of the electrode surface. In Fig. 15, the relative capacitance is plotted as a function of 8,. From this plot, the parameter B in (11) was found experimentally to be 0.45, for black electrodes. When 0 < GL=G1 and CHp > Cpt, the value of B should be greater than 1. Therefore, the presented model of mercury-drop deposition does not describe the behaviour of a platinum-black electrode. The value B -C 1 actually means thatin contrast to the poisoning of smooth electrodes by Mode l-the total surface area of a black electrode is decreased by poisoning. We suppose that mercury drops are formed on platinum-black electrodes as well; they grow in the vicinity of the orifices of pores and shield the whole internal surface of the pore. In this way, a rather small amount of mercury can inhibit hydrogen evolution on a relatively large platinum surface and replaces this extensive area by a much smaller area of mercury drops. So the great efficiency of poisoning and the decrease of the electrode capacitance by the poisoning of platinum black electrodes can be explained. Quite different is the behaviour of a smooth electrode poisoned by Mode 2. According to Fig. 5, just one atom of mercury substitutes one atom of hydrogen and a partial or a complete monolayer of mercury is formed in this way. Hence, the total surface area and also the electrode capacitance remain unaffected by poisoning.
REFERENCES 1. J. Vondrak and J. Balej, Electrochim.
Acta
15, 1653
(1970). 2. J. Vondrak and J. Balej, Electrochim. Acta 16, 1331 (1971). 3. A. I. Shlygin, Utchen. zapis. Kazach. Gos. Univ. 13, 24 (1951). 4. G. P. Khomtchenko and G. D. Vovtchenko, Yesf. Moskov. Univ. No. 8, 115 (1953). 5. G. P. Khomtchenko, Utchen. zapis. Moskov. Gos. Univ. 174, 319 (1955). 6. G. P. Khomtchenko, Vest. Moskov. Univ., ser. MatKhim. 11, 175 (1956). 7. G. P. Khomtchenko, Vesf. Moskov. Univ. Ser. MatKhim. 11, 179 (1956). 8. 3. J. Kulakova and A. V. Shashkina, Zh. fiz. Khim. 35, 1031, 1198 (1961). 9. J. J. Kulakova and A. V. Shashkina, Vesr. Moskov. Univ., Ser. Khim. No. 1, 48, No. 3, 36 (1963). 10. A. V. Shashkina and J. J. Kulakova, Zh.jiz. Khim. 36 393 (1962); 37, 1966 (1963). 11. I. M. Kolthoff and C. S. Miller, J. Am. them. Sot.
63, 1013 (1941). 12. M. W. Breiter, Transactions of the Symposium on Electrode Processes, p. 307. ed. E. Yeager, Wiley, New York (1961). 13. A. D. Semjonova, T. G. Fedotova and G. P. Khomtchenko. Vest. Moskov Univ. Ser. Khim. No. 5. 48 (1965).
54
J. VONDRAK
14. A. N. Frumkin, in Advances in Electrochemistry and Electrochemical Engineering, Vol. 1, p. 3, ed. P. Delahay and C. W. Tobias. Interscience, New York (1961). 15. R. Parsons, Handbook of EIectrochemical Constants, p. 96. Butterworths, London (1959).
AND
J. BALEJ
16. R. Neeb, Inverse Pobzrographie and Voitammetrie. Verlag Chemie, Weinheim (1969). 17. J. F. Strachan and N. Z. Harris, J. Inst. Metals 85, 17 (1956-7). 18. J. Balej and I. Paseka, Chemie Ingr. Tech. 39, 725 (1967).
Acra.
1973,Vol. 18,
pp. 45-54.
Pergamon Press. Printed in Great Britain.
INFLUENCE OF MERCURY ON HYDROGEN OVERVOLTAGE ON SOLID METAL ELECTRODES-III. ADSORPTION AND EVOLUTION OF HYDROGEN ON POISONED PLATINUM J. VONDRAK and J. BALEJ Institute
of Inorganic
Chemistry,
Czechoslovak
Academy
(Received 17 January
of Sciences, Prague, Czechoslovakia
1972)
Abstract-The adsorption capacity for hydrogen as well as the exchange cd of hydrogen evolution and the double-layer capacitance of a platinum electrode depend on the degree of poisoning by mercury. Mercury electrodeposited with simultaneous hydrogen evolution on smooth platinum forms droplets, the curvature radius of which is of the order low6 cm, and for complete inhibition of hydrogen adsorption, an amount equivalent to several monolayers of mercury is necessary. By submerging a pure platinum electrode in a dilute solution of mercuric compounds without external polarization, a monolayer is formed and one atom of mercury replaces just one atom of hydrogen. On platinum-black electrodes, electrodeposited drops of mercury cover the orifice of the pores and one mercury atom shields on an average 2-3 active sites of the electrode surface.
Hydrogen evolution on solid metal electrodes is inhibited by the presence of small amounts of mercury on the electrode surface. According to previous papers[l, 21 amounts of mercury as great as 4 monolayers are necessary for complete poisoning of a smooth platinum electrode. Oh the contrary, Shlygin[3], Khomtshenko et a&&7], and Shashkina and Kulakova[fl-101 found that one atom of mercury is able to displace several atoms of hydrogen from the surface of black platinum and palladium electrodes. To explain this contradiction, the influence of the amount and the manner of deposition of mercury on hydrogen evolution, hydrogen adsorption, and the double-layer capacitance of both black and smooth platinum electrodes was studied. The results of this investigation are the subject of the present paper. EXPERIMENTAL Apparatus,
electrodes
TECHNIQUE
and chemicals
The experiments were carried out in a glass vessel with three compartments. The compartment for the investigated electrode contained only 6 ml of solution saturated either by nitrogen or by hydrogen. It was separated from the other compartments by means of stopcocks. The investigated, auxiliary, and reference electrodes were made from 5 x IO x 0.1 mm Pt foil and were welded to Pt wire, which was sealed in a glass holder with a ground joint. The reference
electrode was covered by platinum black and permanently saturated with hydrogen. Before each series of experiments, the smooth electrodes were successively purified in carbon tetrachloride, nitric acid (about 50 per cent), hydrochloric acid (about 30 per cent) and redistilled water. Before each run, the electrode was cleaned electrochemically by repeated polarization to f1200 mV and 0 mV(he) for a total time of 3 s. From hydrogen adsorption a roughness factor, measurements, f = 6.0 k O-3, was found for electrodes prepared in this way. Several electrodes were covered by platinum black in a solution containing 25 g HZPtCls and 0.08 g Pb(CH3C00)2 in 100 ml Hz0 using a cd of 0.03 A/cm2 for 30 min. Then the electrodes were leached severai days in redistilled water. From hydrogen adsorption measurements in pure 1 N KOH, roughness factors of 223, 315, 325 and 1050 were found. Properties of all the electrodes are summarized in Table 1. Both the hydrogen and the nitrogen used in the electrolytic vessel were purified by means of deoxidation catalysts, silica gel, and cooling with solid carbon dioxide. Both gases were let into the vessel through all-glass and elastic tube. No grease was used for the ground joints. Concentrated sulphuric acid (purity p.a.) and a 50 per cent solution of potassium hydroxide (purity for semiconductors) were used for solution preparation. Both H2S04 and KOH were diluted to a 45
J. VONDRAKAND J. BALEJ
46
Table 1. Properties of individual electrodes used in experiments
Electrode
Electrolyte solution
smooth4
1 NKOH
Roughness factor f
Exchange cd mA/cm’ “* io 10
Double-layer capacitance pF/cm’ c (3
0.48
0.08
85.7
8.7
1.74
110
18.3
14.3
6.0 i- O-3 1 nH2S04 black I
1 NKOH
223
20
0.09
3460
15.5
black II
1NKOH
315
15
O-048
4500
14.3
black III
1NKOH
258
10.5
o-047
5400
21.0
black IV
I N HzS04
14000
13.0
1090
-
* i; = iO]f. t c’ = C/J $ average of five samples.
concentration of 1 N. Mercuric oxide was prepared by precipitation of mercuric nitrate solution by potassium hydroxide. The mercury was scrubbed in dilute nitric acid and distilled in vacuum before preparation of the mercuric nitrate solution. Carbon tetrachloride and nitric and hydrochloric acids were purified by distillation. Four-times distilled water was used for all purposes. Before each series of experiments, the electrolytic vessel was cleaned with a 10 : 1 mixture of concentrated sulphuric acid and a 30 per cent solution of hydrogen peroxide (purity p.a.) and then rinsed with water. The vessel was then filled with the solution, oxygen was removed from the compartment for the investigated electrode, and the solution was purified by pre-electrolysis, using an electrode similar to that investigated and a cathodic cd of 0-l mA/cmZ for 10 h. Electrode
trode was inserted in the measuring vessel. Mercury ions on the electrode surface were reduced to the metal in this way. Tt was found that the rate of poisoning is controlled by the diffusion of Hgz+ ions to the electrode surface. Evidence of this is shown in Fig. 1, in which the charge, Q, necessary for the oxidation of mercury on the electrode surface (see below) is plotted against the square root of time, t, for which the electrode was submerged in 1 N KOH containing 50 mg/l of Hg2+. If each mercury atom is adsorbed immediately after reaching the electrode surface, then the total amount of mercury adsorbed at time t is 2~(Dr#/~, where c is the btllk concentration of Hg2+ ions in the solution and D is their diffusion coefficient. If this
poisoning
Three different modes of poisoning were used for smooth platinum electrodes. Mode 1. A clean electrode was connected to the negative terminal of a constant current power supply and then it was submerged in an electrolytic vessel containing a mercury pool anode and a solution of 1 N potassium hydroxide saturated with mercuric oxide. After time sufficient for deposition of the required amount of mercury, the electrode was withdrawn from the vessel, rinsed with water and transferred into the measuring vessel. Mode 2. A clean electrode was submerged in a solution of 1 N KOH saturated with mercuric oxide, or in a mixture of one volume of this solution and two volumes of pure 1 N KOH. After rinsing with water, the electrode was connected to a potentiostat on which the reversible potential of the hydrogen electrode had been pre-set, and the elec-
Fig. 1. Dependence of the charge required for the oxidation of mercury on the electrode surface on the time of dipping of the electrode in 1 N KOH containing HgO. 0, I N KOH; 0,l N H,SO,.
Influence of mercury on hydrogen overvoltage on solid metal electrodes-III. amount is reduced to metallic mercury, Q is required for its oxidation, Q = 2zF~(Dt7r)“~,
then charge (1)
where z is the va!ency state of the mercury oxidation products. It follows from Fig. 1 that the amount of mercury expressed as the charge Q is proportional to 2/i as required by (1). Further, the slopes of this relationship are different for the oxidation of adsorbed mercury in acidic and alkaline media, ie, 10.5 x 10m5 and 22 x 1O-5 A s’/‘/cm’, respectively. Theoretical values based on the value D = 8.2 x 10m6 cm2/s for mercuric ion in dilute nitric acid[ll] are 8.1 x lo-’ and 16.2 x 10n5 A s’/‘/cm’ for z = 1 and z = 2, respectively. From the ratio of the experimental slopes and from their approximate agreement with the calculated values, two conclusions can be drawn. First, the oxidation of mercury is an oneelectron process in sulphuric acid, while it is a twoelectron process in KOH. Second, the poisoning of an unpolarized electrode in a dilute solution of Hg”+ in 1 N KOH is a diffusion-controlled process. Mode 3. A clean electrode was submerged in liquid potassium amalgam for 1 min, and an almost continuous film of mercury was formed on the surface. Electrodes covered by platinum black were poisoned only according to Mode 1. Experimental
l-5
of potential changes during each experimental run is schematically shown in Fig. 2. After cleaning the electrode by anodic pulses and reduction of oxygen layers on theelectrode (Fig. 2, Section 1), the electrode was poisoned in a separate vessel (Section 2) and transferred back to the measuring cell. The polarization curve of hydrogen evolution was then measured using a step-wise change of potential from 0 to -100 mV(he) in the case of smooth platinum electrodes and to - 60 mV in the case of platinum black electrodes (section 3). In this potential range, the rate of evaporation of mercury from the electrode surface is negligible[2]. Then the double-layer capacitance was measured by superimposing an ac voltage on the dc potential in the range from +300 mV to +500 mV (Section 4) and both ac potential and current were recorded on an X-Y recorder (ENDIM 2200/I) or on an oscilloscope (Tesla OPD 250), and the capacitance was calculated from the Lissajous figures. Finally, the amount of adsorbed hydrogen and mercury were measured from cathodic and anodic voltammetric curves obtained with a voltage of triangular wave-form generated by a low-frequency generator (ARITMA GNK). For all experiments, a potentiostat IP 410 B was used. All potentials are referred to the reversible hydrogen electrode in the same solution. Experiments were done at 20 f 1°C. RESULTS
routine
Preliminary experiments showed that the described experimental technique ensured that the electrode surface remained clean for at least 3 min after electrode purification by anodic and cathodic pulses. Therefore, an experimental routine was chosen so that all electrode parameters, ie the hydrogenevolution rate, the double-layer capacitance, the amount of adsorbed hydrogen and the amount of mercury were measured within 3 min. The programme
47
Hydrogen
adsorption
on poisoned
electrodes
In Fig. 3, a typical anodic branch of a voltammetric curve (the ascending part of section 5 in
--____________--____----------
I _________________
V o-5
0
-05
Fig. 2. Potential changes during one experimental run. 1, electrode cleaning, 2, poisoning in separate vessel; 3, measuring of hydrogen overvoltage; 4, capacitance measurement by superimposed a.c. voltage; 5, cathodic and anodic voltammetry for measuring the amounts of hydrogen and mercury. EA Vol. 18 No. 1-D
Potential Fig. 3. Anodic branch of the voltamrnetric curve of a .. smoorn Pt electrode in 1 N HzS04 at the scanning rate 3 v/s. Full line, clean electrode; dotted line, electrode partly poisoned by mercury.
48
J. VONDRAK AND J. BALEJ
0
2
I P
Fig. 4. Influence of the relative amount of mercury P on the degree of coverage by hydrogen 8, for smooth electrodes poisoned by Mode 1. l,l N KOH; 0,l N HzS04. Fig. 2) for both clean and partly poisoned smooth electrodes is plotted. In these experiments, a scan rate of 3 V/s was used. The curve for the clean electrode is in agreement with that given by Breiter[lZ]. In the presence of mercury on the electrode surface, the amount of adsorbed hydrogen is lower and the peak currents of both hydrogen waves are reduced approximately in the same ratio. Also, the growth of the oxygen layer in the potential range from +0*7 + 1.0 V(he) is slower on a poisoned electrode; this effect is more apparent at higher scanning rates. At potentials more positive than +1-O V, a new wave of mercury oxidation occurs. This wave was also described by Khomtshenko[l31. The amount of adsorbed hydrogen atoms, Q”, was evaluated as an average of the charges of both cathodic and anodic waves on voltammetric curves in the potential range from 0 to +O-4 V, and the degree of coverage by hydrogen was calculated as the ratio of the amount of hydrogen on poisoned (Q”) and clean (Qh) electrodes: 0 = QH/Q~ . The amount of mercury was calculated from the charge QoXabsorbed by the electrode if the potential increased from +0.7 to +l*O V. It was expressed as the ratio of the number of mercury atoms on a poisoned electrode to the number of hydrogen atoms on a clean electrode,
where QZXis the charge required for oxidation of a clean electrode in the same potential range, and the meaning of z is the same as in (1). Its value was taken as z = 1 for acidic solutions and z = 2 for alkaline solutions. For a complete monolayer of mercury, the relative amount of mercury, P = 1, provided that the number of atoms in the monolayers of mercury and hydrogen are the same.
P
Fig. 5. Influence of the relative amount of mercury P on the degree of electrode coverage by hydrogen 6, for smooth electrodes poisoned by Mode 2. .,I N KOH, 0,l N HzS04.
Fig. 6. Influence of the relative amount of mercury P on the degree of electrode coverage by hydrogen & for black electrodes poisoned by Mode 1. Full points, 1 N KOH; open points, 1 N H1S04. v, electrode 1; 0, electrode 2; A., electrode 3 ; 0, electrode 4.
Influence of mercury on hydrogen overvoltage on solid metal electrodes-III.
49
In Figs. 4, 5 and 6, the degree of surface coverage by hydrogen is plotted against the relative amount of mercury, P. In these pictures, the dotted lines, cor-
responding
to substitution
of one hydrogen
atom
by one atom of mercury, are given for comparison. If a smooth electrode is poisoned by Mode 1 (Fig. 4), then small amounts of mercury have almost no influence on hydrogen adsorption, so that several atoms of mercury are necessary for the substitution of one hydrogen atom. This ratio decreases with increasing amounts of mercury, but for complete inhibition of hydrogen adsorption, a large excess of mercury is required. The behaviour of a smooth electrode poisoned by Mode 2 is different (Fig. 5). For relative amounts of mercury, P < O-5, one atom of mercury can substitute just one atom of hydrogen. At higher relative amounts of mercury, poisoning becomes less effective and the ratio between the increment of mercury amount and the decrement of hydrogen coverage is slightly higher than 1, analogous to the previous case. Small amounts of mercury deposited on a platinum black electrode according to Mode 1 (Fig. 6) are a
very effective poison, since one atom of mercury substitutes two atoms of adsorbed hydrogen, if 8, > 0.5. With increasing amounts of mercury, the effectiveness of the mercury decreases; nevertheless, relative amounts, P, as low as 2 are sufficieht for complete inhibition of hydrogen adsorption on platinum black electrodes (ie for 0, -C 0.05).
Fig. 8. Influence of electrode coverage by hydrogen on the relative exchange cd of hydrogen evolution on smooth electrodes poisoned by Mode 2. is approximately proportional to the coverage by hydrogen. This is shown in Figs. 7 and 8 for smooth electrodes poisoned by Mode 1 and Mode 2 and in Fig. 9 for black electrodes poisoned by Mode 1. In these pictures, exchange cds are expressed as the ratio of the exchange cds on poisoned and clean electrodes under the same conditions.
Hydrogen overvoltage on poisoned electrodes Analogous to hydrogen adsorption, the hydrogenevolution rate also decreases with increasing amounts of mercury. The exchange cd for hydrogen evolution
Fig. 9. Influence of electrode coverage by hydrogen on the relative exchange cd of hydrogen evolution on black platinum electrodes.
Fig. 7. Intluence of electrode coverage by hydrogen on the relative exchange cd of hydrogen evolution on smooth electrodes poisoned by Mode 1. .,lNKOH; 0,l NH,SO,.
The poisoning of a smooth electrode by Mode 1 was found to be less effective than poisoning by Mode 2 (cf Figs, 4 and 5, and also previous papers [l, 21). Therefore, the inhibition of hydrogen evolution by mercury must be studied up to amounts of mercury much higher than that equivalent to one monolayer. The dependence of the polarization resistance and the exchange cd is plotted in Fig. 10 against the amount of mercury on the logarithmical
J. VONDRLKAND J. BALEJ
50
Capacitance of poisoned electrodes Amc~lgomated ___________
electrode ___a---
While the differential capacitance of smooth electrodes poisoned by Mode 2 was found to be independent of P in the whole studied range 0 < 8 < 1, within 5 per cent accuracy, the capacitance of both smooth and black electrodes poisoned by Mode 3 depends on the relative amount of mercury, as shown in Figs. 12 and 13 respectively. The capacitances are given in both figures in relative values as the ratio between the capacitance of poisoned and clean electrodes under similar conditions. In Fig. 12, the dotted lines indicate the value of the relative capacities for both clean electrodes and electrodes covered with mercury film (poisoning by Mode 3).
Fig. 10. Influence of the relative amount of mercury on the polarization resistance and the exchange cd of hydrogen evolution on smooth electrodes poisoned by Mode 1. .______ scale. Adsorption of mercury becomes undetectable by sweep voltammetry (0 G 0.03) if the amount of mercury increases to P = 2.7; this point is indicated by an arrow in Fig. 10. The dotted lines in Fig. 10 indicate both the limiting values of the hydrogenevolution rate, ie for a clean electrode and for an electrode poisoned by submerging in potassium amalgam (Mode 3) and covered with an almost continuous mercury film. According to this picture, even amounts of mercury as high as P = 100 do not suppress the hydrogen-evolution rate to the limiting rate found for electrodes covered with a mercury film. Figure 11 shows the dependence of the transfer coefficient /3 on the relative amount of mercury. Also in this picture, limiting values for both clean and mercury-covered electrodes are given.
Pure electrode
-0’0
.
z 0
Amlrlgamated
electrode
l
3
Fig. 12. Influence of relative amount of mercury on the relative electrode capacitance of smooth electrode poisoned by Mode 1.
I
1
P
Fig. 11. Influence of relative amount of mercury on the transfer coefficient B on smooth electrodes poisoned by Mode 1.
Fig. 13. Influence of the relative amount of mercury on the relative electrode capacitance of platinum-black electrodes. Symbols as in Fig. 6.
Influence of mercury on hydrogen overvoltage on solid metal electrodes-III. DISCUSSION According to Figs. 4,s and 6, the relative exchange cd of hydrogen evolution is approximately equal to the degree of coverage by hydrogen, Lel
10,pois e “. = -= i,, E~can
(3)
The total apparent cd on a partly poisoned electrode is the sum of the currents of hydrogen evolution on the clean and the poisoned parts of the electrode surface (supposing that the presence of mercury does not change the electrode surface area), i0. pOis= i,, P, . 0” + io, PfHpl* (1 - &I.
(4)
Here, io. Pt and io, ptHpXare the exchange cds on pure platinum and on platinum covered by mercury. For the relative current density of hydrogen evolution it follows that io,re,=(l-*)
-OH+%.
(4a)
According to Fig. 10, the ratio io, PfHpx/io,Pt = 7.4 x 10-4. Therefore. the last term in (4a‘l is negligible and this equation reduces to (3), as‘foknd experimentally. Equations (3) and (4a) are exactly valid if only one of the following conditions is fulfilled. Either the electrode surface is homogeneous and all centres have the same activity, or the surface is inhomogeneous, but different parts of the surface are poisoned to the same extent irrespective to their activity. The latter seems more probable as on a platinum electrode surface, at least two different types of active centres are present[l2, 141. As will be shown below, we suppose that, in the poisoning smooth electrodes by Mode 1, small drops of mercury are deposited. Owing to the different interfacial energies of the boundaries Pt/Hg, Pt/H, , and Pt/aqueous-solution, it seems probable that mercury drops occupy the concave parts of the surface, while hydrogen bubble formation is easier on the convex parts of the surface. Therefore, the mercury is initially deposited predominantly on parts of the surface that are less active for hydrogen evolution, and the small deflexion of the observed curve (Fig. 7) from (3) can be explained in this way. The exchange cd on an electrode covered by a mercury film (poisoning by Mode 3) is approximately 3 x lo-’ A/cm2 (Fig. lo), while the exchange cd on liquid mercury is 10-l” A/cmz[15]. Thus, a platinum electrode covered by mercury is by 6 orders of magnitude more active than pure mercury. Two explanations of this are possible. First, mercury dissolves platinum, forming amalgams with concentrations up to 0.06-0.1 per cent[16, 171, and the hydrogen overvoltage on platinum amalgam may be lower than that on mercury. Second, platinum is
51
incompletely wetted by mercury; the mercury film on the electrode surface is not continuous and contains small islets of bare platinum acting as hydrogenevolution centres. Actually, one can observe with the naked eye that bubbles of hydrogen occur on almost the same places on the surface. The relative exchange cd, 7-4 x 10e4, on mercury-covered platinum would therefore correspond to the fraction of uncovered platinum surface. Similarly we suppose that incomplete wetting of platinum by mercury is also important at lower mercury coverage on electrodes poisoned by Mode 1. In our previous papers[l, 21, we suggested a hypothesis according to which mercury deposited on a platinum electrode consists of small isolated droplets rather than of a continuous layer. This hypothesis seems sufficiently confirmed by the results of the present paper. Let us suppose for simplicity that the platinum electrode is covered by mercury drops of uniform size and with the shape of a spherical segment with radius of curvature, r, and wetting angle, 4 (Fig. 14). 1 cm2 of physical surface area contains n such droplets.
Fig. 14. Size and shape of a mercury drop on the electrode surface. Let tc = 1 - cos $.
(5)
Then, the volume of one drop is V = 1/3rrr3a2(3 - a), the surface of the spherical segment is S = Tar’ and the surface of the base of the drop is A = rrz * (2a - a’). Since the spherical surface of the drop is greater than its base, the total electrode surface area is increased by poisoning. Further, if d is the density of mercury and m. = 4.46. 10m7 g/cm2 is the amount of mercury required for a monolayer, the relative amount of mercury is given by
P=nY$. The degree of coverage by hydrogen equals part of the electrode not covered by mercury,
eH = I - d.
that (7)
J. VONDRAK AND J. BALEJ
52
Finally, the capacitance of the electrode is the sum of the capacitances of the free platinum surface and of the spherical surface of mercury so that the relative capacitance Gel is given by crc, = 1 - nA + F
P,
* ns,
(8)
where CHI and CP, are the specific capacitances of mercury and platinum respectively. Supposing that n is constant and that with increasing amounts of mercury, only the drop radius increases, from (6) and (7) it follows that
0
112
()
P=&- f
3-a . (2 _ a)3,2 * (1 - e”Y.
(9)
Similarly, if r is constant, and, with increasing amounts of mercury, new drops of constant radius are formed, then
(10)
,
Oo-
eti
f
Fig. 15. Relation between the electrode coverage by hydrogen and the relative electrode capacitance. Symbols as in Fig. 4 and 6. Data given by Khomtshenko for 1 N NaCl + O-01 N NaCl are presented by crosses.
By inserting the values of Q and Sin (8) we obtain the relation for the relative electrode capacitance, C,,, = F Pt . &+e. =B+([email protected]”
1
l-z-$-
a1 111)
This equation does not contain the drop radius r, and should be valid generally if the wetting angle is constant. In Fig. 15, the relative capacitance of both smooth and black electrodes are plotted against 6,. For comparison, data given by Khomtchenko[&7] are also plotted in the same picture. For both electrodes, (11) is valid if 8” > O-1. The parameter, B, for smooth electrodes is l-40, while for a platinum black electrode B is 0.45. From B = 1.40 for a smooth electrode, the wetting angle, 4 = 26.6” was established using (11) and (5) and CP, = 1.43 x 10e5 F/cm2 (found for a clean electrode, see Table l), and C,, = 1.80 x 1O-5 F/cm2 (found for an electrode poisoned by Mode 3). The behaviour of platinum-black electrodes will be discussed separately. From the theory of the condensation of liquids it follows that the formation of a new drop nucleus is more difficult than the growth of a drop previously formed. Hence, nucleation of drops should occur mainly at the beginning of the poisoning process. The number of mercury drops is probably influenced by the properties of the platinum surface and is approximately independent of the relative amount of mercury. Under these conditions, (9) should be valid and the dependence of P on (1 - &) should be a straight line with slope 3/2 on the logarithmic scale. As Fig. 16 shows, this is true only for mercury amounts giving P -=z1. For greater amounts of mercury, the slope decreases to l/2 and it increases again to higher values when most of hydrogen is removed from the electrode, viz for 0, < 0.2. The deviation of the observed curve from (9) in the medium range of mercury coverage is probably caused by the increasing number of drops in this range, while in the range of the highest amounts of mercury, drops of mercury fuse to greater lakes, which no longer have the shape of a spherical segment. Moreover, this more or less continuous mercury film also fills the micro-roughness of the electrode surface. For these reasons, not even (11) is valid and the relative capacitance decreases up to a value close to the reciprocal of the roughness factor (Fig. 15). From Fig. 16 it follows, for example, that 8” = 0.9 for P = 0.6. For this particular coverage, the number of drops, n = 2.2 x IO” cm-‘, and the drop radius, r = 1.92 x 10e6 cm, were calculated using 4 = 26.6” and (9) and (7). The vapour pressure on a curved mercury surface with this curvature radius is 1.325 times higher than that on a plane mercury surface. It was found experimentaIly that the mercury evaporation rate is
53
Influence of mercury on hydrogen overvoltage on solid metal electrodes-III.
P
I-
01
I
l-8, Fig. 16. The relation between P and (1 - 19,) for smooth electrodes poisoned by mode 1. The dotted line has slope 3/2.
1.14 f @05 times higher than that calculated for the equilibrium vapour pressure of pure mercury[2]. Hence, the vapour pressure elevation and theincrease in the mercury evaporation rate also coniirm the presented hypothesis of mercury-drop formation on a poisoned platinum surface. The difference between the theoretical and observed vapour-pressure elevation may be caused partly for the reasons discussed in the previous paper, partly by the fact that theevaporation rate was measured at higher amounts of mercury and therefore at greater radius of curvature of the mercury drops. According to a previous paper[l], the Tafel slope b of the hydrogen-evolution reaction on a posioned electrode is higher than on clean electrodes. This is in agreement with Fig. 11, which shows that, with increasing amounts of mercury, deposited by Mode 1, the transfer coefficient t?~decreases. This phenomenon can also be explained by the presence of mercury drops. The wetting angle, 4 on the platinum/ mercury/aqueous-solution boundary decreases if the potential increases to more negative values[ll], so that the mercury drops should spread and a greater fraction of the platinum surface would be shielded by the same amount of mercury than at less negative potentials. Therefore, the decrease of the transfer coefficient /3is only apparent. The behaviour of a black platinum electrode poisoned by Mode 1 is different. According to Fig. 6, one atom of mercury replaces two atoms of hydrogen. Similarly, Khomtchenko[4-71 found that one atom of mercury replaces 2-7 atoms of hydrogen on a platinum electrode with a specific area of surface 10,600. According to Shashkina and Kulakova[8-101 one atom of mercury replaces 3.1 atoms of hydrogen on black palladium electrodes with a specific surface area of 500. This phenomenon has usually been
explained by the heterogeneity of the electrode surface and by preferential poisoning of the most active centres of the electrode surface. In Fig. 15, the relative capacitance is plotted as a function of 8,. From this plot, the parameter B in (11) was found experimentally to be 0.45, for black electrodes. When 0 < GL=G1 and CHp > Cpt, the value of B should be greater than 1. Therefore, the presented model of mercury-drop deposition does not describe the behaviour of a platinum-black electrode. The value B -C 1 actually means thatin contrast to the poisoning of smooth electrodes by Mode l-the total surface area of a black electrode is decreased by poisoning. We suppose that mercury drops are formed on platinum-black electrodes as well; they grow in the vicinity of the orifices of pores and shield the whole internal surface of the pore. In this way, a rather small amount of mercury can inhibit hydrogen evolution on a relatively large platinum surface and replaces this extensive area by a much smaller area of mercury drops. So the great efficiency of poisoning and the decrease of the electrode capacitance by the poisoning of platinum black electrodes can be explained. Quite different is the behaviour of a smooth electrode poisoned by Mode 2. According to Fig. 5, just one atom of mercury substitutes one atom of hydrogen and a partial or a complete monolayer of mercury is formed in this way. Hence, the total surface area and also the electrode capacitance remain unaffected by poisoning.
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Acta
15, 1653
(1970). 2. J. Vondrak and J. Balej, Electrochim. Acta 16, 1331 (1971). 3. A. I. Shlygin, Utchen. zapis. Kazach. Gos. Univ. 13, 24 (1951). 4. G. P. Khomtchenko and G. D. Vovtchenko, Yesf. Moskov. Univ. No. 8, 115 (1953). 5. G. P. Khomtchenko, Utchen. zapis. Moskov. Gos. Univ. 174, 319 (1955). 6. G. P. Khomtchenko, Vest. Moskov. Univ., ser. MatKhim. 11, 175 (1956). 7. G. P. Khomtchenko, Vesf. Moskov. Univ. Ser. MatKhim. 11, 179 (1956). 8. 3. J. Kulakova and A. V. Shashkina, Zh. fiz. Khim. 35, 1031, 1198 (1961). 9. J. J. Kulakova and A. V. Shashkina, Vesr. Moskov. Univ., Ser. Khim. No. 1, 48, No. 3, 36 (1963). 10. A. V. Shashkina and J. J. Kulakova, Zh.jiz. Khim. 36 393 (1962); 37, 1966 (1963). 11. I. M. Kolthoff and C. S. Miller, J. Am. them. Sot.
63, 1013 (1941). 12. M. W. Breiter, Transactions of the Symposium on Electrode Processes, p. 307. ed. E. Yeager, Wiley, New York (1961). 13. A. D. Semjonova, T. G. Fedotova and G. P. Khomtchenko. Vest. Moskov Univ. Ser. Khim. No. 5. 48 (1965).
54
J. VONDRAK
14. A. N. Frumkin, in Advances in Electrochemistry and Electrochemical Engineering, Vol. 1, p. 3, ed. P. Delahay and C. W. Tobias. Interscience, New York (1961). 15. R. Parsons, Handbook of EIectrochemical Constants, p. 96. Butterworths, London (1959).
AND
J. BALEJ
16. R. Neeb, Inverse Pobzrographie and Voitammetrie. Verlag Chemie, Weinheim (1969). 17. J. F. Strachan and N. Z. Harris, J. Inst. Metals 85, 17 (1956-7). 18. J. Balej and I. Paseka, Chemie Ingr. Tech. 39, 725 (1967).